In automotive body construction there is an increasing trend to use a unitary structure, which may be cast or molded, in preference to an assembled stack of sheet metal components. The cast or molded unitary structure may, if a casting, be a cast iron but is more often of low-density material, aluminum or magnesium which will, in addition to simplifying assembly, enable vehicle mass saving. Molded structures may be polymers, most commonly reinforced polymers. Such a unitary body however must be integrated into and joined with the remainder of the body structure, possibly requiring that a non-ferrous casting of aluminum or magnesium alloy or a molded reinforced polymer be joined to sheet steel or sheet aluminum. Such a joint is typically made using a mechanical fastening process like self-piercing riveting or SPR.
Self-piercing riveting (SPR) is a mechanical joining process for attaching two or more workpieces to one another. Unlike many riveting processes it does not require that holes be predrilled to accommodate the rivet, a feature enabling flexible part fit-up and making it an attractive process for automobile body assembly. A self-piercing rivet is a hollow metal cylinder closed on one end by an integral cap which overhangs the cylinder diameter. The open end of the rivet is driven into a stack of workpieces until the underside of the cap contacts the surface of the stack. The SPR process is typically performed cold, that is, at room temperature or about 25° C., and the stack may comprise two or more workpieces. The total stack thickness is limited to no more than about 12 millimeters or so, and the rivet length is selected so that it fully penetrates all but the last workpiece in the stack which the rivet penetrates only partially. The stack is supported by a shaped die in contact with the last workpiece in the stack. Under the influence of the die and the pressure applied to drive the rivet into the stack, the rivet is upset and the cylindrical walls of the rivet are radially expanded and splayed outward to create mechanical interference between the rivet and the pierced cavity, locking all the workpieces of the stack together.
SPR is commonly used on wrought ductile sheet materials since the piercing action of the rivet during penetration and the upsetting results in significant local deformation of the workpieces. Application of SPR to workpieces of more limited ductility, such as castings or reinforced polymers, and more particularly to magnesium alloy castings, could result in cracking of the workpieces which would reduce the joint strength and the holding power of the rivet.
There is thus a need to adapt the assembly of vehicle incorporating thin, cast metal or molded polymer structural components with reduced ductility to accommodate the SPR process for vehicle assembly.